1. Field of the Invention
The invention relates to a signal processing in object detection.
2. Description of the Background Art
Conventionally, there has been a technology [e.g. Adaptive Cruise Control (ACC)] that enables a vehicle on which a radar apparatus is mounted (hereinafter referred to simply as “vehicle”) by detecting a position, etc. of a vehicle traveling in front (hereinafter referred to as “front vehicle”) of the vehicle by using the radar apparatus, to follow the front vehicle, keeping a predetermined distance. The ACC is used, for example, when the vehicle follows the front vehicle traveling in a longitudinal distance of approx. 100 m ahead of the vehicle traveling on a highway, or when the vehicle follows the front vehicle in a longitudinally short distance of 4 m from the vehicle traveling in traffic congestion, keeping an inter-vehicular distance of approx. 2 to 4 meters from the front vehicle at a low speed (e.g. 10 km/h or less). Herein, the term “longitudinal distance” means a distance that a transmission wave transmitted from a transmitter antenna of the radar apparatus travels to an object or that a reflected wave of the transmission wave reflected by the object travels back to a reception antenna of the radar apparatus. The longitudinal distance is hereinafter simply referred to as distance.
However, if the distance between the vehicle and the front vehicle is short, there is a case where the reception antenna of the radar apparatus receives a multiply-reflected wave caused between the vehicle and the front vehicle by multiple reflection of the wave transmitted from the transmitter antenna of the radar apparatus to detect the front vehicle. In addition, there is a case where the transmission wave does not only reflect by a vehicle body of the front vehicle, but reflects by a base part of the vehicle body and a road surface, etc. by going into a space between the base part of the vehicle body and the road surface, and thus the reception antenna of the radar apparatus receives such a reflected wave. As a result, there has been a case where a signal processor of the radar apparatus fails to pair a peak signal in an up-modulating interval with a peak signal in a down-modulating interval because no corresponding peak signal exists, or where a mis-pairing occurs because the signal processor pairs a peak signal with a non-corresponding peak signal.
Herein, the term “peak signal(s)” refers to a signal(s) of which strength is greater than a predetermined threshold among a plurality of signals generated per frequency after the signal processer of the radar apparatus processes beat signals, by using Fast Fourier Transform (FFT), obtained by mixing transmission signals and reception signals.
Then the signal processor determines whether or not a pair data set has been derived in a current object detection process based on an object on which a pair data set had been derived from a past object detection process. If the signal processor determines that these pair data sets have been derived based on the same object, these pair data sets are used as successive pair data sets.
Herein, the term “a pair data set(s)” refers to data generated by pairing the peak signal in the up-modulating interval with the peak signal in the down-modulating interval. The pair data set mainly includes information of a relative distance between the vehicle and an object, a relative speed between the vehicle and the object, and an angle of the object viewed from the vehicle.
The following is an example of concrete conditions to determine whether or not the pair data set has time continuity from another pair data set. Herein, the term “to have time continuity from” refers to a situation in which a data set is a successive data set to or succeeds another data. First, the signal processer has derived a pair data set in a most recent (preceding) object detection process (hereinafter also referred to as “preceding pair data set”) among past object detection processes, and derives a predicted pair data set that will be derived in the current object detection process (hereinafter also referred to as “current pair data set”), based on an object data derived in the preceding object detection process (hereinafter also referred to as “preceding object data set”) based on the preceding pair data set.
Then the signal processor determines that the current pair data has the time continuity from the preceding pair data if a difference in the relative speed between the predicted data set and an actual current pair data set is, for example, 2 km/h or less. Then, the signal processor generates an object data set of the object processed in the current object detection process (hereinafter also referred to as “current object data set”) based on the current pair data set and the preceding object data set, and outputs the current object data set to a vehicle controller that controls each of apparatus, devices, etc. of the vehicle.
In other words, the signal processor deems the current object data set having the time continuity as a data set corresponding to the same object for which the preceding object data set derived in the past, and assigns to the current object data set a same target No. assigned to the preceding object data set, and then outputs the current object data set to the vehicle controller. At least one of the preceding object data set and the current data set may be hereinafter referred to simply as “object data set.”
If the relative speed difference of 2 km/h or less that is a condition to determine whether or not the pair data set has the time continuity, is changed, a problem shown below occurs. Concretely, if a difference between a prediction relative speed for the current pair data set predicted based on the preceding object data set and an actual relative speed included in the current pair data set is changed from 2 km/h or less to a value greater than 2 km/h (e.g. 10 km/h or less), a problem as shown below occurs.
For example, if the current pair data set is derived from a mis-paired peak signal, even when the front vehicle that is an object corresponding to the current pair data set is stopping, there may be a case where the signal processor implements a following process based on a result of determination of the time continuity. In other words, there is a case where the signal processor derives an incorrect relative speed of the front vehicle based on the pair data set derived from the mis-paired peak signal, and outputs to the vehicle controller the current object data set indicating that the front vehicle travels at a relative speed difference of 10 km/h or less (e.g. 10 km/h) as compared to a relative speed derived in a preceding object detection process.
As a result, there is a possibility that the vehicle starting traveling collides with the front vehicle that is actually stopping when the vehicle controller controls the vehicle to follow the front vehicle. Therefore, because of a possible occurrence of a pair data set derived from a mis-paired peak signal, a condition of a relative speed difference needs to be set in a range as narrow as possible. Concretely, a relative speed difference of 5 km/h or less is more preferable than a relative speed difference of 10 km/h or less, and a relative speed difference of 2 km/h or less is more preferable than the relative speed difference of 5 km/h or less.
However, if a relatively small difference value (e.g. relative speed difference of 2 km/h or less) between the prediction relative speed for the current pair data set predicted from the preceding object data set and the relative speed included in the actual current pair data set is continuingly used as the condition of determination of the time continuity, when the front vehicle starts to move at a speed that makes the relative speed difference more than 2 km/h, e.g. the relative speed difference of 5 km/h, the signal processor determines that the current pair data set has no time continuity from the preceding object data set. As a result, the current pair data set becomes a pair data set having no time continuity and is detected for the first time (newly detected) in the current object detection process. Thus an object indicated by the current pair data set is regarded to be located further than a location of the stopping front vehicle detected in the preceding object detection process (a location further from the vehicle than a location where the front vehicle had stopped). Then a signal processor outputs to the vehicle controller a current object data set corresponding to the current pair data set, from a radar apparatus.
Moreover, in such a case, although the current pair data set newly derived in the current object detection process has no time continuity from the object data derived in the preceding object detection process, the signal processor outputs to the vehicle controller the object data set corresponding to the front vehicle indicating that the front vehicle is stopping in front of the vehicle (at a same location where the front vehicle has been detected in the preceding object detection process). In other words, the signal processor implements a process (hereinafter referred to as “extrapolation process”) for treating the object that is not detected in the current object detection process as an object that were locating in front of the vehicle (at a location where the front vehicle had stopped), in object detection processes implemented after the current object detection process.
As a result, as compared with the current pair data set corresponding to the front vehicle, actually travelling, detected as a new object, a data set corresponding to the front vehicle detected by the extrapolation process (hereinafter referred to as “extrapolation data set”) is closer to the vehicle. Therefore, the object indicated by the extrapolation data set becomes a following target. Therefore, although the front vehicle is actually traveling, the vehicle remains stopping because there is no change in the location of the object corresponding to the extrapolation data set. Thus, there is a possibility that the controlling of the vehicle to follow the front vehicle cannot be implemented fully.